Thermal behaviours of high-performance digital circuits in bulk CMOS and FDSOI technologies are compared on a 64-bit Kogge-Stone adder designed in 40nm CMOS node. Temperature profiles of the adder in bulk and FDSOI are extracted with thermal simulations and hotspot locations are studied. The influence of local power density on peak temperature is examined. It is shown that high power density devices have significant influence on peak temperature in FDSOI. It is found that some group of devices that perform the same function are the most prominent heat generators. A modification on the design of these devices is proposed which decreases the hotspot temperatures significantly.